To get this achievement really easily, refrain from buying anything from the shop (EXCEPT FOR THE REQUIRED ITEMS NEEDED TO PROCEED) from when you start the game to when you start the inverted castle. If you still do not have the achievement when you start the inverted castle, look in your options menu to see how much you have.
Try selling any gems you'd found in the regular castle to the shop (Located in the library), you should have it then.